1. Field of Invention
This invention pertains generally to the field of radio frequency metal detectors, and more particularly to the calibration of such a device.
2. Description of Prior Art
Metal detectors are used in the food processing industry, for example, to detect contaminants within a product. The unwanted material may include very small metallic particles having differing compositions. As seen in FIG. 2, the typical metal detector is housed in an enclosure 26 containing a longitudinal aperture 25 through which the product under test 23 is transported, usually by means of a conveyor belt, in the direction of arrow 24. The metal detector includes a radio frequency transducer or oscillator that radiates a magnetic field by means of some arrangement of coils that serve as a radio frequency antenna. An example of such a metal detector operating in the radio frequency range is disclosed in U.S. Pat. No. 5,994,897, entitled FREQUENCY OPTIMIZING METAL DETECTOR, issued on Nov. 30, 1999 to King.
The typical metal detector enclosure 26 includes both radiating and receiving coils formed to surround the aperture 25 through which the product travels. The oscillator coil is a continuous wire loop formed within the search head. The oscillator coil surrounds the aperture 25 and receives radio frequency excitation from an oscillator circuit. The enclosure 26 also includes an input coil connected to produce a zero input signal when no metal is present.
A disturbance in the radiated magnetic field is sensed by the input coil and processed in order to detect a metal contaminant within the product passing through the detector aperture. Modern digital signal processing techniques resolve the input signal into two signal components, one component being resistive and the other signal component being reactive. FIG. 1 depicts a typical signal processing scheme used in such a metal detector. The coils 1 are connected to the search head 2 that contains a radio frequency transmitter and receiver. When the coils 1 receive an electromagnetic signal the search head 2 divides the received signal into a reactive (X) component 11 and a purely resistive component 12. The signals 11 and 12 are in an analog form and so are forwarded to the analog to digital (A/D) converter 3 where the signal 11 is converted into a digital reactive component signal 13 and a digital resistive component signal 14. An example of a metal detector using digital signal processing techniques is disclosed in U.S. Pat. No. 7,432,715, entitled METHOD AND APPARATUS FOR METAL DETECTION EMPLOYING DIGITAL SIGNAL PROCESSING, issued on Oct. 7, 2008 to Stamatescu.
A nonzero input coil signal is due to either mechanical imbalances in the construction of the search head, inherent electrical changes in the circuitry such as frequency drift, metal being introduced into the aperture, or the effect of the product itself. The “product effect” is caused by the product passing through the aperture and is due primarily to electrical conduction via salt water within the product, the electrical conduction causing large magnitude resistive signals and relatively smaller reactive signals.
Calibration of a metal detector including compensation for the effect of the product is usually accomplished by the user of the detector. This process is dependent on operator skill and experience, and results in inconsistent results between different operators using the same machine. An example of a manually operated interactive metal detection calibration process is disclosed in U.S. Pat. No. 6,816,794, entitled APPARATUS AND METHOD FOR DETECTING CONTAMINATION OF OBJECT BY A METAL, issued on Nov. 9, 2004 to Alvi.
An attempt to directly address the effect of the product is disclosed in U.S. Pat. No. 6,636,827, entitled FOREIGN MATTER DETECTOR AND FOREIGN MATTER DETECTING SYSTEM, which was issued to Sakagami on Oct. 21, 2003. The Sakagami system relies on a library of stored product effect parameters that are manually selected by the equipment operator in order to reduce the sensitivity of the metal detector to the effect of the product.
A related patent is U.S. Pat. No. 5,045,789, DETECTOR FOR DETECTING FOREIGN MATTER IN OBJECT BY USING DISCRIMINANT ELECTROMAGNETIC PARAMETERS, issued on Sep. 3, 1991 to Inoue, et al, which discloses the concept of defining a set of parameters or values which define the detecting envelope, and thus the border between an acceptable product and one containing metal.
U.S. Pat. No. 4,719,421, entitled METAL DETECTOR FOR DETECTING PRODUCT IMPURITIES, issued on Jan. 12, 1988 to Kerr discloses the use of an adjustable phase shifter (element 10 in FIG. 2). The phase shifter is adjusted to provide a null or linear output in response to product variations that might otherwise erroneously indicate the presence of metal. More specifically, if a nonlinear output is produced more than a given number of times in succession, thereby indicating that the nonlinearity is characteristic of that particular product, then the phase shifter is adjusted to produce a linear output.
There are numerous disadvantages to the metal calibration methods just described. In general, the operator of the metal detector does not have a clear understanding of the concept and function of the metal detection process. This lack of understanding leads to misuse of detection calibration controls and necessarily to a reduction in metal detector sensitivity due to improper settings of the metal detector system. A need therefore exists to employ a method of metal detector product effect compensation which permits substantially all product effect corrections to be performed automatically by the metal detector.